As is well known, particulates such as fly ash, sulfur oxides, and/or other contaminants must be removed from combustion exhaust gases in order to comply with federal, state and possibly local emission requirements. These gases usually result from the combustion of coal or other fossil fuel and they often are emitted from power generating facilities, waste-to-energy plants, and/or other industrial processes.
Known methods of accomplishing sulfur and other contaminant removal include utilizing fossil fuels having a low content of such contaminants. Lacking such a supply of fuel or in addition to the use of such fuel, sulfur and/or contaminant content can also be reduced prior to combustion via mechanical or chemical processes. One major disadvantage of such mechanical or chemical processing is the cost required to achieve the desired level of sulfur or contaminant removal.
A method of removal occurring after combustion involves mixing dry alkali material either with the fuel prior to combustion or injecting such material directly into the hot combustion gases after combustion has occurred. This will enable the then available alkali to remove sulfur oxides and other contaminants via absorption or adsorption followed by oxidation. Unfortunately, this method fouls the heat transfer surfaces which necessitates more frequent soot blowing. Other disadvantages include low to moderate removal efficiencies, poor reagent utilization, and increased particulate loadings in the combustion gases. This last disadvantage may also require further conditioning (i.e. humidification or sulfur trioxide injection) of the combustion gas if an electrostatic precipitator is used for downstream particulate collection.
Alternately, a method known as wet chemical absorption (i.e. wet scrubbing) may be used. In accordance with this method, the hot combustion gases are typically washed in an up flow gas liquid contact device with an aqueous alkaline solution or slurry. Such solution or slurry chemically alters and removes the sulfur oxides and other contaminants from the flue gases. The disadvantages of this method include the loss of liquid both to the atmosphere (i.e. due to saturation of the flue gas and mist carry-over) and the sludge produced during this process. Additional disadvantages pertain to the cost associated with the material needed for the construction of the absorber module and all related downstream auxiliary equipment (i.e. primary/secondary dewatering and waste water treatment subsystems).
The method known as "dry scrubbing" (i.e. spray drying chemical absorption process) may also be utilized to remove sulfur oxides and other contaminants from the flue gases. In accordance with dry scrubbing, an aqueous alkaline solution or slurry is finely atomized (via mechanical, dual fluid, or rotary cup type atomizers) and sprayed into the hot flue gas. This contact chemically alters the sulfur oxides and other contaminants in the flue gas thereby leading to their removal. The major disadvantages to dry scrubbing being a moderate to high pressure drop across the spray dryer gas inlet distribution device and the limitation on temperature drop allowed in order to maintain controlled operations.
While the above methods pertain primarily to sulfur oxide and other contaminant removal, they do not necessarily address the removal of particulates such as fly ash and the like. To remove these particulates, a baghouse or an electrostatic precipitator (ESP) is often utilized.
A baghouse typically incorporates one or more fabric filters that trap particulate matter (dust) by direct impingement on the filter cloth. This method may be used as a means of dry particulate collection where the temperature and humidity of the gases to be handled are within an acceptable range. Typical types of baghouses include pulse-jet and reverse air types and such baghouses may often be positioned around the chimney through which the exhaust gases exit the facility.
Electrostatic precipitators utilize an electric charge that is applied to the particulate matter (dust) to be collected. This charge subsequently propels the particles, by electrostatic force, to the collecting curtain where they are removed from the flue gas. Such a method may be used provided the temperature and humidity of the flue gases are within an acceptable range. Typical types of electrostatic precipitators include rigid electrode, rigid frame and weighted wire types.
It is an object of this invention to provide a means of removing particulates in addition to sulfur oxides and other contaminants from exhaust flue gases in an efficient and economical manner. A main object of this invention is to combine a dry scrubber and a particulate collection device into a single integrated unit. Another object of this invention is to eliminate the need and thus the cost of ductwork normally required to transfer the flue gases between the two devices. Still another object of this invention is to configure a baghouse or other type of particulate collection device around a vertical co-current down flow dry scrubber. Yet another object of this invention is to save space at the facility by combining these two operations into a single piece of equipment. While the prior art discusses flue gas desulfurization systems in series with particulate collection devices, an object of this invention is to combine these components so that they become integral parts of the same vessel, device, or apparatus. Still another object of this invention is to incorporate these two devices together and install it in the unused space at the base of the chimney. These and other objects and advantages of this invention will become obvious upon further investigation.